Synthesis of substantially monodispersed colloids

ABSTRACT

A method of forming ligated nanoparticles of the formula Y(Z) x  where Y is a nanoparticle selected from the group consisting of elemental metals having atomic numbers ranging from 21-34, 39-52, 57-83 and 89-102, all inclusive, the halides, oxides and sulfides of such metals, and the alkali metal and alkaline earth metal halides, and Z represents ligand moieties such as the alkyl thiols. In the method, a first colloidal dispersion is formed made up of nanoparticles solvated in a molar excess of a first solvent (preferably a ketone such as acetone), a second solvent different than the first solvent (preferably an organic aryl solvent such as toluene) and a quantity of ligand moieties; the first solvent is then removed under vacuum and the ligand moieties ligate to the nanoparticles to give a second colloidal dispersion of the ligated nanoparticles solvated in the second solvent. If substantially monodispersed nanoparticles are desired, the second dispersion is subjected to a digestive ripening process. Upon drying, the ligated nanoparticles may form a three-dimensional superlattice structure.

RELATED APPLICATION

[0001] This application is a division of Ser. No. 09/977,838, filed Oct.15, 2001.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] This invention was made with government support under Grant No.NAG8-1687 awarded by NASA. The government has certain rights in theinvention.

BACKGROUND OF THE INVENTION

[0003] 1. Field of the Invention

[0004] The present invention is concerned with methods of forming largequantities of ligated nanoparticles which can be deposited in two- andthree-dimensional superlattices. Broadly speaking, the method involvesinitially forming a first colloidal dispersion made up of nanoparticlessolvated in a molar excess of a first solvent, a second solventdifferent from the first solvent, and a quantity of ligand moieties.Thereupon, a substantial part of the first solvent is removed and theligand moieties are caused to ligate to the nanoparticles to give asecond colloidal dispersion. Preferably, the second dispersion issubjected to a heat and reflux digestive ripening process to givesubstantially monodispersed colloidal particles. The invention alsopertains to the ligated nanoparticle colloidal dispersions and to thefinal products.

[0005] 2. Description of the Prior Art

[0006] It is known that high surface area nanoparticles can be formed bya vaporization-co-condensation process sometimes referred to as thesolvated metal atom dispersion (SMAD) method. The latter involvesvaporization of a metal under vacuum and codeposition of the metal atomswith the vapors of a solvent on the walls of a reactor cooled to 77 K(liquid nitrogen temperature). After warm-up, nanoparticles arestabilized both sterically (by salvation) and electrostatically (byincorporation of negative charge). The SMAD technique was firstdisclosed in 1986 by Klabunde and co-workers, and is also described inU.S. Pat. No. 4,877,647. A major advantage of the SMAD process is thatno biproducts of metal salt reduction are present, and pure metalcolloids are formed. Additionally, the SMAD process lends itself toindustrial-scale operations, as opposed to other competing processessuch as the inverse micelle and reductive procedures for metal colloidpreparation.

[0007] Organization of nanoparticles into two and three-dimensionalstructures (nanocrystalline superlattices, NCSs) leads to the formationof materials characterized by very different properties compared tothose of the discrete species. The manifestation of novel andtechnologically attractive properties is due to the collectiveinteractions of the particles, as well as to the finite number of atomsin each crystalline core. Synthesis and characterization of suchmaterials are interesting from both fundamental and industrial points ofview. Regularly arranged nanosized particles find applications in thedevelopment of optical and electronic devices, and magnetic recordingmedia, for example. Nanoparticles of gold and other noble metals haveattracted significant attention not only because of ease of preparation,but also due to their potential application in nano andmicroelectronics. Heretofore the challenge has been to form a structureof a planar array of small metal islands separated by tunnel barriersfor use in electronics. Gold nanoparticles are excellent candidates inthis respect.

[0008] Numerous methods for synthesis of particles arranged in 2D- and3D-NCSs have been reported in the literature. The most common proceduresinclude reduction of a suitable metal salt in the presence of differentstabilizing agents. In all methods, the most important requirement isthe ability to produce monodispersed particles that can order over along-range. Crystalline arrays of particles covered by organic moleculeshave become of great interest, especially since the improved synthesisof thiol-stabilized gold nanoparticles has been developed (Brust, etal., J. Chem. Soc., Chem. Commun., 1994, 801-802). Their advantage isthat they behave as simple chemical compounds in respect that they canbe dissolved, precipitated, and redispersed without change inproperties, much as molecular crystals can.

SUMMARY OF THE INVENTION

[0009] The present invention is broadly concerned with methods offorming ligated nanoparticle colloidal dispersions and recovered ligatednanoparticles which may be in superlattice form. In general, the methodinvolves initially forming a first colloidal dispersion comprisingnanoparticles solvated in a molar excess of a first solvent, a secondsolvent different than the first solvent, and a quantity of ligandmoieties. Next, a substantial part of the first solvent is removed andthe ligand moieties are caused to ligate to the nanoparticles to give asecond colloidal dispersion comprising the ligated nanoparticlessolvated in the second solvent. If desired, the ligated nanoparticlesmay then be recovered as a dry product which, depending upon the natureof the nanoparticles and ligands selected, may assume a superlatticeconfiguration.

[0010] Preparation of the first colloidal dispersion is preferablyaccomplished by vaporizing the solid substance (e.g., metal or metalsalt) and first solvent in a reactor to give vaporized atoms ormolecules and depositing the vaporized atoms or molecules and firstsolvent onto a cold surface. Upon subsequent warming of this mixture,nanoparticles are formed by aggregation of the atoms or molecules, andthese nanoparticles and first solvent are allowed to mix with a secondsolvent and ligand moieties. Thereupon, the first solvent is removed byvacuum, which substantially completely eliminates the first solvent andalso, to a limited degree, some of the second solvent.

[0011] In a particularly preferred technique, the second colloidaldispersion is subjected to a digestive ripening process so that thevariation in particle size of the ligated nanoparticles is reduced; thisripening process is advantageously carried out until the second colloidis essentially monodispersed. This ripening process is also important ifa superlattice dry product is desired.

[0012] The nanoparticles useful in the invention are generally selectedfrom the group consisting of the elemental metals having atomic numbersranging from 21-34, 39-52, 57-83 and 89-102, all inclusive, the halides,oxides and sulfides of such metals, and the alkali metal and alkalineearth metal halides. Elemental gold and silver are particularlypreferred, with elemental gold being the single most preferrednanoparticle material. The nanoparticles should have an average diameterof from about 2-50 nm, and more preferably from about 3-15 nm.Similarly, the nanoparticles should have a BET surface area of fromabout 15-500 m²/g, and more preferably from about 50-300 m²/g.

[0013] The first and second solvents should be selected so that thefirst solvent may be readily removed by vacuum distillation or othertechniques from the initial colloid. In practice, the first solventshould have a boiling point at least about 25° C. (more preferably atleast about 40° C.) below the boiling point of the second solvent. Ofcourse, the first and second solvents must also have the ability tosolvate the nanoparticles and ligated nanoparticles, respectively.

[0014] Although a wide variety of solvents may be employed, preferablythe first solvent is a ketone, and especially a ketone selected from thegroup consisting of ketones of the formula

[0015] where R₁ and R₂ are independently and respectively selected fromthe group consisting of straight and branched chain C1-C5 alkyl andalkenyl groups, and the C1-C5 straight and branched chain alcohols. Thesingle most preferred first solvent is acetone. The first solvent shouldbe used at a level so that it is in molar excess relative to thenanoparticles, and preferably a molar excess of from about 50-1000should be established.

[0016] The second solvent is preferably an aryl organic solvent such atoluene or xylene. More broadly, the solvent is advantageously selectedfrom the group consisting of solvents of the formula

[0017] where X₁ and each X₂ are each independently and respectivelyselected from the group consisting of H and C1-C5 straight and branchedchain alkyl and alkenyl groups, n is from 0 to 3, each X₂ may beindependently located at any unoccupied ortho, meta or para positionrelative to X₁.

[0018] A variety of ligands may be used in the invention, and can beatoms, ions, or compounds. As used herein “ligand moieties” refers toall such ligand species. The preferred class of ligands are thiolcompounds selected from the group consisting of compounds

R₃SH

[0019] where R₃ is a C5-C20 straight or branched chain alkyl or alkenylgroup. More preferably, R₃ is a C10-C15 straight or branched alkyl oralkenyl group; an especially preferred ligand is dodecanethiol.

[0020] The digestive ripening process comprises the step of heating andrefluxing the second colloidal dispersion, preferably at a temperatureof from about 60-180° C. under an inert gas such as argon for a periodof from about 10-400 minutes. The goal of digestive ripening is toreduce the particle size variation in the ligated nanoparticles;preferably, this process is carried out to achieve a ligatednanoparticle surface area of up to about 20% above and below the meansurface area of the ligated nanoparticles.

[0021] The final ligated nanoparticles in general have the formulaY(Z)_(x) where Y is the nanoparticle and Z is the ligand; x is variabledepending upon the nanoparticle and ligand selected. In the case of thepreferred Au(dodecanethiol) ligated nanoparticles, x would typicallyrange from about 300-10,000, with a ligand density on the goldnanoparticle surface ranging from about 1-10 ligand moieties per squarenanometer of nanoparticle surface area.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 is a flow diagram illustrating the synthetic steps in apreferred method for the preparation of nanocrystal superlatticeproducts;

[0023]FIG. 2 is an TEM micrograph of gold particles in anAu-acetone-toluene-thiol colloid (colloid 1) described in the Example;

[0024]FIG. 3A is a TEM micrograph of gold particles in anAu-toluene-thiol colloid (colloid 2) described in the Example;

[0025]FIG. 3B is another TEM similar to that of FIG. 3A, but viewinganother area of the TEM grid;

[0026]FIG. 4 is a graph depicting the UV/VIS absorption spectra ofas-prepared colloid 2 (solid line) and the digestively ripened colloid(dotted line);

[0027]FIG. 5A is a TEM micrograph of gold particles after the digestiveripening step in the Example, where sampling was done from the hotcolloidal dispersion;

[0028]FIG. 5B is another TEM photograph similar to that of FIG. 5A;

[0029]FIG. 6A is TEM micrograph of gold particles after the digestiveripening step of the Example (sampled from hot dispersion);

[0030]FIG. 6B is a histogram derived from the measurement of 400particles which corresponds to the gold particle TEM micrograph of FIG.6A;

[0031]FIG. 7A is a TEM micrograph of gold particles 15 minutes after thecompletion of the digestive ripening process of the Example;

[0032]FIG. 7B is a TEM micrograph similar to that of 7A, but depictingthe gold particles one day after the completion of the digestiveripening process of the Example;

[0033]FIG. 7C is another TEM micrograph similar to that of 7B, anddepicting the gold particles one day after the completion of thedigestive ripening process of the Example; and

[0034]FIG. 7D is a TEM micrograph similar to that of 7A, but depictingthe gold particles approximately two months after the completion of thedigestive ripening process of the Example.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0035] The following example sets forth presently preferred methods forthe preparation of ligated nanoparticle superlattices in accordance withthe invention. It is to be understood, however, that this Example isprovided by way of illustration and nothing therein should be taken as alimitation upon the overall scope of the invention.

[0036]FIG. 1 is a flow diagram of the most preferred preparation ofgold-containing nanocrystalline superlattice products. This method isalso explained in detail below.

EXAMPLE

[0037] A) Preparation of Au-acetone-toluene-thiol Colloid (Colloid 1).

[0038] A stationary reactor described in Klabunde, et al., Inorg. Syn.,Shriver, D., ed., 19, (1979), 59-86 was used for the synthesis ofAu-acetone-toluene-thiol colloid. Acetone and toluene solvents werepurchased from Fisher. Acetone was dried over molecular sieve. Bothacetone and toluene were degassed five times by the standard freeze-thawprocedure prior to the reaction. Dodecanthiol was purchased from Aldrichand used as received. All glassware was rigorously cleaned before use.

[0039] A W-Al₂O₃ crucible was assembled in the SMAD reactor and thewhole system was pumped down. This was followed by a step-wise heatingof the crucible and the pressure was allowed to reach 4×10⁻³ Torr ateach heating step. The crucible was heated to red in about half an hour,then the heating was decreased and the whole reactor was left undervacuum overnight while the crucible was gently heated. This processensured no contamination of the crucible. After the overnight treatment,the reactor was filled with air and the crucible was charged with ˜0.3 gAu metal. At the same time 8 ml (6.8 g, 3.4×10⁻² mol) of dodecanethiolwas placed in the bottom of the reactor chamber together with a stirringbar. Degassed acetone and toluene solvents were placed in Schlenk tubesand attached to the SMAD reactor. The whole system was evacuated and aliquid nitrogen filled Dewar placed around the vessel. Dodecanethiol wasfrozen in this way in the bottom of the reactor. When the vacuum reached4×10⁻³ Torr, 40 ml of toluene was evaporated in ˜15 min and frozen onthe walls of the reactor. The liquid nitrogen Dewar was removed andtoluene allowed to melt undisturbed and fall to the bottom of thereactor. The liquid nitrogen Dewar was again put in place, and Au vapor(0.27 g, 1.4×10⁻³ mol) and acetone (100 ml) were codeposited over aperiod of 3 hours. During this time, the pressure was maintained atabout 4×10⁻³ Torr. The frozen matrix had a deep red color at the end ofthe deposition. After the process was complete the liquid nitrogen Dewarwas removed and the matrix allowed to warm slowly over a period of ˜1hour. During the warmup process argon gas was allowed to fill thereactor system. Upon melting the Au-acetone matrix mixed with thetoluene and the color became deep brown. When the dodecanethiol startedto melt, stirring was started and the whole solution was agitated foranother 45 min. The as-prepared dark brown Au-acetone-toluene-thiolcolloid (colloid 1) was syphoned under argon into a Schlenk tube.

[0040] B) Preparation of Au-toluene-thiol Colloid (Colloid 2).

[0041] The Schlenk tube containing the as-preparedAu-acetone-toluene-thiol colloid (colloid 1) was connected to a vacuumline and the acetone was evaporated until a constant 1×10⁻² Torrpressure was reached (the more volatile acetone was removed along withsome of the toluene). At this time the Au-toluene-thiol colloid wasdiluted to 80 ml by addition of degassed toluene. Thus the total volumeof the final dark brown Au-toluene-thiol colloid was 80 ml containingabout 0.20 g of gold.

[0042] C) Digestive Ripening.

[0043] The digestive ripening process is an important step for formationof a monodispersed colloid from the polydisperse Au-toluene-thiolcolloid (colloid 2). The procedure involved heating under reflux of acertain amount of Au-toluene-thiol colloid for 1.5 hours. The heatingtemperature is the boiling point of the colloidal solution (˜120° C.).The digestive ripening was carried out under an argon atmosphere.

[0044] D) Isolation of a Dry Product.

[0045] Isolation of a dry product was done after the gold-toluene-thiolcolloid (colloid 2) was subjected to digestive ripening for 1.5 h. Aftercooling down to room temperature, 10 ml of the digested colloid(containing 0.025 g Au) was precipitated with 50 ml of absolute ethanol.After overnight treatment, the precipitation was complete and thesupernatant was carefully removed by sucking out with a Pasteur pipette.The remaining precipitate together with a small amount of leftovertoluene, thiol and ethanol was dried under vacuum until constantpressure (5×10⁻³ Torr). After drying, the color of the product wasbrown-red and it had the appearance of a wet paste. An additional 3 mlof ethanol was added and the system was left undisturbed overnight. Thesupernatant was then removed and the sediment again was dried undervacuum at constant pressure. After drying the precipitate (0.0214 g) wasa powder with small shiny-dark crystals. It was washed again with 3 mlof ethanol, left overnight, the supernatant removed and dried undervacuum. After drying, the precipitate was 0.0207 g and no change of themass was recorded after additional washing with ethanol and drying undervacuum. The yield was 84% based on gold. If the adsorbed thiol is takeninto account, the yield was ˜73%.

[0046] The final dry product was in the form of soft, shiny darkcrystals, which are readily soluble in toluene or hexane. After additionof the solvent, the crystals immediately dissolved giving wine-redcolored colloidal solution. However, the crystals are not soluble inethanol or acetone.

[0047] E) UV-VIS Spectroscopy.

[0048] UV/VIS absorption spectra were obtained using a Fiber Optic CCDArray UV-VIS Spectrophotometer of Spectral Instruments, Inc.

[0049] F) Transmission Electron Microscopy (TEM).

[0050] TEM studies were performed on a PHILIPS CM100 operating at 100kV. The TEM samples were prepared by placing a 3 μl drop from thecolloidal solution onto a carbon coated formvar copper grid. The gridswere allowed to dry in air for 1 hour and left undisturbed at ambientconditions.

Results and Discussion

[0051] Since the first report in 1986 (Lin, et al., Langmuir, 2, (1986),259-260) of the synthesis of nonaqueous colloidal gold solutions by theSMAD method, considerable work has been carried out on the preparationand characterization of several non-aqueous metal nanoscale particles(Franklin, et al., High-Energy Processes in Organometallic Chemistry;Suslick, K. S., ed., ACS Symposium series, (1987), 246-259; Trivino, etal., Langmuir, 3, 6, (1987), 986-992). Colloidal solutions of gold inacetone have been one of the most intensively studied andwell-understood systems. Acetone, as a polar solvent, solvates the metalatoms and clusters during the warmup stage. In this way stericstabilization is achieved and gold colloids are stable for months.

[0052] These earlier results were the motivation for choosing acetone asan initial solvent in the present example. Preliminary attempts toimprove size-distribution of particles from pure acetone solutions usingthe digestive ripening procedure turned out unsuccessful, and it wasdiscovered that an additional stabilizing agent like dodecanethiol wasneeded. However, when only acetone was used as the solvent, addition ofdodecanethiol did not allow the formation of a stable colloid. Forexample, the precipitate formed after addition of dodecanethiol toAu-acetone colloids, when separated and dried under vacuum, was onlypartially redispersable in toluene. Digestive ripening of the partiallyredispersed Au-colloids led to the size improvement of only thoseparticles that were redispersed. The particles that remained in thesediment did not change their shape and size during this procedure.Therefore, it was found that a combination of solvents such as acetoneand toluene was needed during the SMAD reaction and subsequent clustergrowth and ligation by the thiol. The role of acetone was found to bestabilization of the gold nanoparticles in a preliminary way.

[0053] The size and shape changes of nanoparticles in the differentsamples were investigated by TEM. Representative transmission electronmicrographs of the gold colloids at each step of the preparativeprocedure of the monodispersed colloid are shown in the Figures. A flowdiagram of the major synthetic steps is given in FIG. 1. The resultsfrom the separate preparative stages are discussed below.

[0054] Formation of Monodispersed Thiol-protected Au-colloid

[0055] A) Au-acetone-toluene-thiol Colloid (Colloid 1).

[0056] The initial Au-acetone-toluene-thiol colloid has a dark browncolor. TEM studies of this colloid (FIG. 2) illustrate particles rangingfrom 5 to 40 nm with no definite geometrical shapes. These particles arevery similar to the ones obtained in pure acetone solvent. As reportedin the prior art, two types of stabilization are characteristic forthese systems: 1) steric stabilization (by solvation with the acetonemolecules) and 2) electrostatic stabilization (by acquiring electronsfrom the reaction vessel walls, electrodes, solvent medium). Anotherindication that the gold particles are negatively charged is theoccasionally observed ‘blinking’ in the electron microscope due to theinteraction of the particles with the negatively charged electron beam.However, it should be pointed out that in no case was change in theshape or morphology of the particles observed under the influence of theelectron beam. Both stabilization processes take place during the warmupstep, should to be carried out slowly in order to ensure goodstabilization.

[0057] B) Au-toluene-thiol Colloid (Colloid 2).

[0058] The Au-toluene-thiol colloid (colloid 2) was obtained by vacuumevaporation of all the acetone from colloid 1. TEM micrographs of tworepresentative types of particles found in the colloid are shown inFIGS. 3A and 3B. Drastic change of the size and shape of the particlesis characteristic at this stage. Nearly spherical particles with sizesin the range of 1 to 5 nm are dominant. There are also a small number oflarger particles (10-40 nm) like those in the initial acetone-containingcolloid.

[0059] UV/VIS absorption spectrum (FIG. 4) of colloid 2 is in agreementwith the sizes of the particles observed in TEM. It is characterized bya broad plasmon absorption band with no definite maximum.

[0060] One possible explanation for the change of size and shape of thegold particles induced by the removal of acetone is the following. Incolloid 1 the amount of acetone is in great excess. It strongly solvatesthe gold particles and the attachment of dodecanethiol molecules on theparticles' surface is suppressed. As acetone is removed from the system,the ability for thiol adsorption is increased. Thus acetone acts as apreliminary stabilizing agent, which is substituted by dodecanethiolmolecules when acetone is evaporated. This ensures good dispersity ofthe thiol-ligated gold particles in the toluene medium. The fact thatmost of the particles in the Au-colloid after evaporation of acetonehave size in the region of 1 to 5 nm suggests that some ripening hasalready taken place, presumably due to the strong adsorption ofdodecanethiol molecules on their surface. At this stage the colloid isready for digestive ripening.

[0061] C) Digestive Ripening of Colloid 2 and Organization of the GoldParticles.

[0062] Heating of colloid 2 under reflux results in a dramatic narrowingof the particle size-distribution. TEM studies (FIGS. 5A and 5B) of ahot colloidal solution show formation of spherically shaped particleswith sizes of about 4 nm. They have a tendency to organize into2D-layers. Some of the particles from the hot colloid organize in nice3D-structures. The remarkable effect of the digestive ripening procedureis the great improvement of the size-distribution. Practicallypolydisperse colloid containing particles with sizes ranging from 1 to40 nm are transformed into an almost monodispersed colloid withparticles' sizes of about 4-4.5 nm. A photograph taken at highermagnification (FIG. 6) reveals that the shape of the particles is morepolyhedral rather than spherical. The average size diameter is 4.5 nmand the size-distribution is log-normal as typical for colloidalsystems. The UV/VIS absorption spectrum of the colloid after cooling toroom temperature (FIG. 4) shows an appearance of a definite plasmonabsorption maximum at 513 nm, which is in agreement with the size andmonodispersity of the obtained particles.

[0063] The TEM micrographs of colloids cooled down for a differentamount of time are shown in FIGS. 7A-7D. The amazing result is that theparticles predominantly organize on the TEM grid in large 3D-structuresin only about 15 min after the digestive ripening process is finished(FIG. 7A). A small number of areas of 2D-arrangement is also observed.

[0064] Even larger 3D-structures (>3 μm) are observed after 1 day (FIGS.7B and 7C) and after ˜2 months (FIG. 7D). The results suggest that theactivation energy for 2D-organization is lower compared to this of3D-organization.

We claim:
 1. A three-dimensional supperlattice array of ligatednanoparticles produced by a method comprising the steps of: forming afirst colloidal dispersion comprising nanoparticles solvated in a molarexcess of a first solvent, a second solvent different than the firstsolvent, and a quantity of ligand moieties; removing a substantial partof said first solvent and causing said ligand moieties to ligate to saidnanoparticles to give a second colloidal dispersion comprising theligated nanoparticles solvated in said second solvent; and recoveringsaid ligated nanoparticles.
 2. The superlattice array of claim 1, eachof said nanoparticles comprising a gold nanoparticle with a plurality ofthiol ligand moieties ligated thereto.